SIP in Yate

Yate is a back-to-back user agent (B2BUA) that receives a request and processes it. Yate is not a proxy server. Unlike a proxy server, Yate maintains dialog state and must participate in all requests sent on the dialogs it has established.

SIP (Session Initiation Protocol) is a signaling protocol used to create, manage and terminate sessions in an IP based network. The SIP protocol it can run on Transmission Control Protocol (TCP), User Datagram Protocol (UDP), or Stream Control Transmission Protocol (SCTP).

The flow for this messages is similar to the HTTP request/response transaction model. Yate has implemented a mechanism of transactions. A transaction occurs between a user agent client (UAC) and a user agent server(UAS), and comprises all messages from the initial request to the final response . The responses can be provisional, starting with 1 followed by two digits (for example, "180 Ringing") or final starting with 2 followed by two digits (for example, "200 Ok"). The scope of a transaction is defined by the stack of the Via headers of the SIP messages. A call leg is a dialog. A dialog is a peer-to-peer SIP relationship between two UAs that persists for some time. A dialog is established by SIP messages, such as INVITE to an request a 2xx response. A dialog is identified by a call identifier.

More information: SIP Protocol.